1. Field of the Invention
The present invention relates to a duplexer for use in communication systems such as microwave communication systems, and more particularly, to a duplexer having a laminated structure.
2. Description of the Related Art
A conventional laminated type duplexer is shown in FIGS. 4 and 5. Referring first to FIG. 4, a laminated duplexer 1 includes a laminated structure defined by ceramic sheets 2 to 9. Inductor patterns 12 to 17 are provided on a surface of the ceramic sheet 6. Frequency-adjusting capacitor patterns 18 to 23 are provided on a surface of the ceramic sheet 7. Coupling-adjusting capacitor patterns 24 to 27 are provided on a surface of the ceramic sheet 5. Shield patterns 28a and 29a are provided on a surface of the ceramic sheet 3, and shield patterns 28b and 29b are provided on a surface of the ceramic sheet 9.
The duplexer 1 includes a three-stage band-pass filter BPF1 having LC resonators Q1 to Q3 at the left as viewed in FIG. 4, and a three-stage band-pass filter BPF2 having LC resonators Q4 to Q6 at the right as viewed in FIG. 4. The inductor patterns 12 to 17 define inductors L1 to L6 of the LC resonators Q1 to Q6, respectively. The frequency-adjusting capacitor patterns 18 to 23 and the ends of the inductor patterns 12 to 17 which face the frequency-adjusting capacitor patterns 18 to 23 define capacitors Cl to C6 of the LC resonators Q1 to Q6, respectively.
The LC resonators Q1 to Q3 of the band-pass filter BPF1 are electrically connected to coupling capacitors Cs1 and Cs2 (not shown in FIGS. 4 and 5). The coupling and adjusting capacitors Cs1 and Cs2 are defined by the inductor patterns 12 to 14 and coupling-adjusting capacitor patterns 24 and 25, which face these inductor patterns 12 to 14. The shield patterns 28a and 28b are arranged such that the patterns 12 to 14, 18 to 20, 24 and 25 are positioned therebetween.
Likewise, the LC resonators Q4 to Q6 of the band-pass filter BPF2 are electrically connected to coupling capacitors Cs3 and Cs4 (not shown). The coupling capacitors Cs3 and Cs4 are defined by the inductor patterns 15 to 17 and coupling-adjusting capacitor patterns 26 and 27, which face the inductor patterns 15 to 17. The shield patterns 29a and 29b are arranged such that the patterns 15 to 17, 21 to 23, 26 and 27 are positioned therebetween.
The ceramic sheets 2 to 9 are laminated, and are integrally fired to define a laminate 35 shown in FIG. 5. The laminate 35 is provided with a transmitter terminal electrode Tx, a receiver terminal electrode Rx, an antenna terminal electrode ANT, and grounding terminal electrodes G1 to G4. The inductor pattern 12 of the LC resonator Q1 is connected to the transmitter terminal electrode Tx, and the inductor pattern 17 of the LC resonator Q6 is connected to the receiver terminal electrode Rx. The inductor patterns 14 and 15 of the LC resonators Q3 and Q4 are connected to the antenna terminal electrode ANT. The grounding terminal electrode G1 is connected to one end of each of the inductor patterns 12 to 14, and the grounding terminal electrode G2 is connected to one end of each of the frequency-adjusting capacitor patterns 18 to 20 in the LC resonators Q1 to Q3. The grounding terminal electrodes G1 and G2 are also connected with the shield patterns 28a and 28b. The grounding terminal electrode G3 is connected to one end of each of the inductor patterns 15 to 17, and the grounding terminal electrode G4 is connected to one end of each of the frequency-adjusting capacitor patterns 21 to 23 of the LC resonators Q4 to Q6. The grounding terminal electrodes G3 and G4 are also connected with the shield patterns 29a and 29b. 
In general, duplexers have characteristics that depend upon the Q factor of inductors of LC resonators. The Q factor of an inductor is expressed by Q=2xcfx80f0L/R, where L represents the inductance of the inductor, R represents the resistance of the inductor, and f0 represents the resonant frequency. From the equation, it is clear that the resistance R should be reduced to increase the Q factor of the inductor. The resistance R is inversely proportional to the cross-sectional area S of an inductor pattern that is used to define the inductor. To increase the Q factor of the inductor, therefore, the cross-sectional area S of the inductor patterns 12 to 17 must be increased.
However, increasing the thickness of the inductor patterns 12 to 17 to increase the cross-section S of the inductor patterns 12 to 17 produces undesirable results. Specifically, an internal strain of the laminate 35 is increased causing delamination when the ceramic sheets 2 to 9 are integrally fired. Furthermore, if pattern widths of the inductor patterns 12 to 17 are increased to increase the cross-section S of the inductor patterns 12 to 17, the LC resonators Q1 to Q6 is greatly increased.
The axial directions of the inductors L1 to L6 of the LC resonators Q1 to Q6 are perpendicular to the stacking direction of the ceramic sheets 2 to 9. When an electric current flows through the inductors L1 to L6, a magnetic flux xcfx86 is generated so as to surround the inductors L1 to L6 on planes perpendicular to the axial directions of the inductors L1 to L6. However, since the inductors L1 to L6 and the patterns 18 to 23, 24 to 27, 28a, 28b, 29a and 29b are arranged in parallel, the magnetic flux xcfx86passes through the patterns 18 to 23, 24 to 27, 28a, 28b, 29a and 29b, so that eddy currents are generated in the patterns 18 to 23, 24 to 27, 28a, 28b, 29a and 29b. This produces inductors L1 to L6 that have very low Q factors.
To overcome the above-described problems, preferred embodiments of the present invention provide a laminated-type duplexer which is compact and which has inductors with very high Q factors.
To this end, preferred embodiments of the present invention include a laminated type duplexer having insulator layers which are laminated to define a laminate including a plurality of filters embedded therein, each of the filters having an inductor and a capacitor, wherein each inductor includes a via hole or via-holes connected in sequence in the stacking direction of the insulator layers, and at least two adjacent filters of the plurality of filters are electrically connected to each other through a matching inductor pattern.
Since the inductor is defined by the via-holes connected in sequence, increasing the cross-section of each via-hole or increasing the number of via-holes results in increased cross-sectional area of the inductor. This improves the Q factor of the inductor without increasing the thickness or width of inductor patterns in conventional technique.
When an electric current flows through the inductor, magnetic flux is generated to surround the inductor on a plane that is substantially perpendicular to the axial direction of the inductor. However, since the inductor is substantially perpendicular to a capacitor pattern and a shield pattern, the generated magnetic flux does not pass through such patterns, so that no eddy current occurs in such patterns. This results in an inductor having a very high Q factor and reduced eddy current loss.
Other features, elements, characteristics and advantages of present invention will become apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.